Interesting add-ons that I don't use or have not installed
Use Windows and Internet Explorer Favorites directly from Firefox. No need to import, export or synchronize - the same Favorites appear...
Simple content blocker, will block URLs based on the content of a simple text file in your profile directory (If you don't know the location of your Firefox profile, this is not the extension for you!)
When click on the selected tab, a history menu corresponding to it will display.
Shows source comments for advanced application preferences in about:config
Send a tab or link to another device using Firefox Sync.
Remembers last selected (hilited) extension/theme/plugin/userstyle in Add-ons Manager. Turn off/on the add-on selection list box and/or fine tune behavior via its Options.
Screen Draw lets you draw or type text over the top of any page in any colour or size and then save it to png or jpeg.
Extract Text from HTML code and make the content more readable than code
Adds a Google +1 Button to your urlbar to +1 any page. More on http://www.google.com/+1/button/
Improvement User Script Manager Greasemonkey (based on version 0.922) - Configuration Functionality for User Scripts outside Profile Folder (e.g. in DropBox Folder to share scripts)
Multiple Addon Deactivator (M.A.D) is an addon that can (de)activate one, many or each addon at the same time